Moderate Temperature Curing of Plant Oils with Bismaleimides via the

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Moderate Temperature Curing of Plant Oils with Bismaleimides via the Ene Reaction Brinda Mehta, Paula Watt, Mark Deland Soucek, and Coleen Pugh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03004 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 28, 2016

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Industrial & Engineering Chemistry Research

Moderate Temperature Curing of Plant Oils with Bismaleimides via the Ene Reaction Brinda Mehta,a Paula Watt,a Mark D. Soucek, b and Coleen Pugh* a Departments of Polymer Sciencea and Polymer Engineeringb The University of Akron Akron, Ohio 44325-3909

ABSTRACT. Both soybean oil and linseed oil were cured in bulk with 1,1'-(methylenedi-1,4-phenylene)bismaleimide (MDA-BMI) and oligomeric bismaleimides (BMI-1700 and BMI-3000) at 150 °C for 45 min to yield bio-based thermoset polymers. 1H NMR spectroscopic characterization of a model reaction between soybean oil and N-phenylmaleimide confirmed that the products were formed only by an ene reaction. The curing reactions of bismaleimdes with soybean and linseed oils occurred in the absence of solvent and without added catalyst to produce crosslinked materials. Differential scanning calorimetry demonstrated that curing with MDA-BMI begins at 150 °C when the bismaleimide starts to melt, with maximum curing at 179 °C. The 5% weight loss decomposition temperature of the crosslinked thermosets determined by TGA varied with the crosslinkers used according to: BMI-3000 > BMI-1700 > MDA-BMI. Glass fiber-reinforced composites with 45 wt% glass fibers were prepared by curing a mixture of plant oil and bismaleimide crosslinker applied to a sheet of glass fibers at 150 °C in a press for 10 min. As quantified using dynamic mechanical analysis, the composites based on MDA-BMI were densely crosslinked and stiff, whereas those based on oligomeric BMI-1700 and BMI-3000 were loosely crosslinked and flexible.

The tensile properties of the glass fiber-reinforced

composites varied little with the plant oil used. KEY WORDS: plant oils, soybean oil, linseed oil, renewable resources, ene reaction, bismaleimides, thermosets, composites * To whom correspondence should be addressed.

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Introduction Bismaleimide (BMI) systems are one of the leading resins for reinforced composites for high-performance applications such as aerospace, printed circuit boards, structural laminates and elastomers.1,2,3,4 The well-known advantages of BMI resins include excellent processability, high thermal and mechanical stability, and constant electrical properties over a wide temperature range.

They offer thermal properties, performance properties and price between those of

polyimides and epoxies.5 For commercial purposes, BMIs are usually cured with petroleumbased co-monomers such as epoxies, polyimides and polyesters.5,6,7 The polymerization curing of such co-monomers is never 100% complete and the unreacted monomer tends to leach out of the polymer matrix, which raises environmental and health concerns;8,9 these monomers have been reported to cause skin irritation, cancer, endocrine disruption and many other diseases.10,11 Other disadvantages of these petroleum-based comonomers are their longer biodegradation times and their dependence on synthetically resourced and depleting fossil fuels, which fluctuate dramatically in price. Plant oils and fatty acids offer a new platform of matrix resins for reinforced composites that are bio-renewable and eco-friendly.12,13,14 Plant oil feedstocks are inexpensive and globally and abundantly available. As shown in Figure 1, these triglycerides are mainly composed of saturated and unsaturated fatty acids, with four reactive sites that can be used to introduce polymerizable groups.

Without modification, the internal double bonds are not sufficiently

reactive to readily undergo homopolymerization. Polymers synthesized solely by oxidative curing of plant oils have insufficient mechanical and thermal stability for high performance applications.15

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CH3 H3C O

O O

O

O

O CH3

Figure 1. Triglyceride molecules with four reactive sites: (dotted circles from left to right) carbon-carbon double bonds, ester group, methylene group alpha to the ester and allylic group. The oil extracted from soybeans is a “semi-drying” oil. It contains approximately 23% oleic acid, 53% linoleic acid and 8% α-linolenic acid;

“high oleic” soybean oil with

approximately 81% oleic acid chains has recently been developed through genetic modification.16 Soybean oil is therefore composed primarily of unsaturated and non-conjugated fatty acid chains. Maleic anhydride17 can react with allyl groups by an ene reaction in a process known as “maleinization”,18 which results in addition of maleic anhydride (the enophile) to the olefinic system with an allylic shift of one double bond. Mechanistically, the ene reaction is related to the better known Diels-Alder reaction and was misconstrued as a Diels-Alder reaction until the 1960s.19,20,21 This paper describes the development of crosslinked thermosets by reaction of plant oils with bismaleimides, starting with the bismaleimide derived from 1,1'-(methylene-di-1,4phenylene) (MDA-BMI) (Figure 2), via an ene reaction.

Ene reactions have been used

extensively to functionalize polybutadiene rubber with various enophiles.22,23,24,25,26,27,28,29 Dehydrated castor oil and tung oil have also been crosslinked with MDA-BMI to obtain biobased lubricants.30,31 Various other unsaturated substrates have been cured with MDA-BMI to obtain bio-based thermosetting materials;32,33,34,35,36,37 the decomposition temperatures, glass transition temperatures, and storage moduli increased with increasing MDA-BMI content. In



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most cases, the curing conditions involved the use of a solvent, an inhibitor to suppress radical polymerization of the diene moieties, and extensive post-curing procedures. The unsaturated systems underwent homopolymerization, copolymerization, ene reactions and Diels-Alder reactions simultaneously to obtain the cured material.

Figure 2. Chemical structures of the main reactants used in this study: 1,1'-(methylene-di-1,4phenylene)bismaleimide (MDA-BMI), N-phenylmaleimide (PMI), BMI-1700, BMI-3000 and soybean oil. This paper presents the functionalization and characterization of bio-based thermosetting resins obtained from plant oils using the three bismaleimides shown in Figure 2. The small molecule MDA-BMI should provide tighter crosslinks, whereas the oligomeric BMI-1700 and 4 ACS Paragon Plus Environment

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BMI-300 should produce looser crosslinks; the numbers 1700 and 3000 in the BMI names correspond to their approximate molecular weights. The N-phenylmaleimide monofunctional enophile shown in Figure 2 was used for model reactions to elucidate the reaction mechanism, with characterization of the products by 1H NMR spectroscopy. The thermal and mechanical properties of the crosslinked materials and/or their glass fiber-reinforced composites were characterized by thermogravimetric analysis and tensile testing using an Instron.

Experimental Section Materials.

BMI-1700 (Designer Molecules), BMI-3000 (Designer Molecules), 1,3-

dimethyl-2-imidazolidinone (DMI; Aldrich, 98%), linseed oil (Cargill, Supreme Grade), methyl linoleate (Sigma Aldrich, ≥99% GC), 1,1'-(methylene-di-4,1-phenylene)bismaleimide (Sigma Aldrich, 95%), oleic acid (Eastman Chemical Company, PamolynTM 100 FGK Kosher), Nphenylmaleimide (Sigma Aldrich, 97%), and soybean oil ((SBO, Sigma Aldrich) were used as received. All other reagents and solvents were commercially available and used as received. Techniques. All reactions and polymerizations were conducted under a N2 atmosphere using a Schlenk line unless noted otherwise.

1

H NMR spectra (δ, ppm) were recorded on a

Varian Mercury 500 (125 MHz) instrument. All spectra were recorded in CDCl3, and the resonances were measured relative to residual solvent resonances and referenced to tetramethylsilane (0.00 ppm). A Perkin Elmer Pyris 1 differential scanning calorimeter was used to determine the thermal transitions, which were read as the maximum or minimum of the endothermic or exothermic peaks, respectively. Glass transition temperatures (Tgs) were read as the middle of the change in heat capacity. All heating and cooling rates were 10 °C/min. Transition temperatures were calibrated using indium and tin standards; enthalpy was calibrated



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using an indium standard. The 5% weight loss temperature was measured by thermogravimetric analysis (TGA) using a TA Instrument Model Q500 under nitrogen and air atmospheres at a heating rate of 10 °C/min. The tan delta and storage modulus values were obtained on a Rheometric Scientific DMTA IV instrument using rectangular plates in a cantilever mode with a frequency of 1 Hz and a heating rate of 5 °C/min for the hard composite material and 2 °C/min for the tough flexible material. Tensile testing was performed on ASTM D412-C dumbbell shaped samples using Instron 5567 with a testing speed of 5 mm/min. For each type of molded material, the tensile experiment was repeated three times and the mean values are reported. Model Reaction of Soybean Oil and N-Phenylmaleimide. Table 1 summarizes the ratios of soybean oil and N-phenylmaleimide used for the model reactions. In a typical reaction, a solution of N-phenylmaleimide (1.5 g, 8.7 mmol) and 1,3-dimethyl-2-imidazolidinone (0.3 mL) in soybean oil (1.0 mL, 1.1 mmol, 4.9 mmol C=C) was stirred at 150 °C for 24 h in a 20 mL vial capped under N2. The reaction mixture was then poured into water (10 mL), and the precipitate was collected in a fritted glass filter, washed sequentially with methanol (20 mL), hexanes (20 mL) and water (5 mL), and dried to constant weight in a vacuum oven heated at 65 °C for 24 h to yield 2.0 g (83%) of a brown solid. 1H NMR: 0.88 (m, CH3), 1.26-1.30 (m, (CH2)n), 1.61 (br m, CH2CH2CO2), 2.01-2.06 (m, =CHCH2), 2.29-2.32 (m, CH2CO2), 2.77 (m, =CH-CH2-CH=, CHaHa’CON), 2.93 (br d, CHaHa’CON), 3.3 (d, CHCON), 4.14 (dd, CHaHa’CHO2C, J = 11.7 Hz, 5.9 Hz), 4.29 (dd, CHaHa’CHO2C, J = 11.7 Hz, 4.4 Hz), 5.26-5.40 (m, CHO2C, CH=CH), 7.22-7.24 (br m, aromatic H at o-positions), 7.32-7.45 (br m, aromatic H at m- and p-positions).


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Table 1. Varying Ratios of Soybean Oil (Ene) with N-phenylmaleimide (Enophile) Used in the Model "Ene" Reactions in 1,3-Dimethyl-2-imidazolidinone (Solvent) at 150 ºC for 24 h. Entry

Molar Ratio Ene : Enophile

Product

1

4.6 : 1

dark brown solids

2

2.3 : 1

dark brown solids

3

1.1 : 1

dark brown solids

4

0.6 : 1

brown solids

Curing Reactions Using MDA-BMI as the Crosslinker.

In a typical reaction, a

solution of MDA-BMI (0.40 g, 1.1 mmol) in soybean oil (1.0 mL, 1.1 mmol, 4.9 mmol C=C) in a 20 ml vial capped under nitrogen was stirred at 150 °C for 45 min. After cooling to room temperature, all soluble compounds were extracted into CH2Cl2 from the crosslinked product using a Soxhlet extractor. The residue was dried to constant weight at reduced pressure and room temperature to yield 0.22 g (17%) of a yellow crosslinked solid. As summarized in Table 2, crosslinked solids were similarly obtained using linseed oil. Curing Reactions Using BMI-1700. In a typical example, BMI-1700 (1.6 g, 0.53 mmol) was introduced to a reaction flask as a solution in CH2Cl2 (3 mL), and then the solvent was removed at reduced pressure. Soybean oil (1.0 g, 1.2 mmol, 5.5 mmol C=C) was add to the flask and the resulting solution was stirred at 150 °C for 1.25 h. All soluble compounds were extracted into CH2Cl2 from the resulting gel using a Soxhlet extractor. The residue was dried to constant weight at reduced pressure and room temperature to yield 0.54 g (21%) of an orange



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crosslinked gel. As summarized in Table 2, yellow crosslinked gels were similarly obtained using linseed oil. Table 2. Varying Ratios of Soybean Oil (Ene) with Bismaleimides (Enophile) Used in the Model "Ene" Reactions in Bulk at 150 ºC for 45 min. Entry

Ene : Enophile

Molar Ratio Ene : Enophile

Molar Ratio [C=C bonds] : [maleimide groups]

Product

P1

Oleic Acid : MDA-BMI

1:1

0.5 : 1

Cream Solids

P2

Methyl Linoleate : MDA-BMI

1:1

1:1

Yellow Solids

P3

Soybean Oil : MDA-BMI

0.5 : 1

1.2 : 1

Yellow Solids

P4

Linseed Oil : MDA-BMI

0.5 : 1

1.5 : 1

Yellow Solids

P5

Soybean Oil : BMI-1700

2:1

4.6 : 1

Yellow Gel

P6

Linseed Oil : BMI-1700

2:1

6:1

Yellow Gel

P7


1:1

2.3 : 1

Yellow Gel

P8


0.5 : 1

1.2 : 1

Yellow Gel

P9


1:1

3:1

Yellow Gel

P10


0.5 : 1

1.5 : 1

Yellow Gel

Curing Reactions Using BMI-3000 as the Crosslinker. In a typical reaction, a solution of BMI-3000 (1.7 g, 0.55 mmol) in soybean oil (0.5 mL, 0.55 mmol, 2.2 mmol C=C) in a 20 mL vial capped under nitrogen was stirred at 150 °C for 3 h. All soluble compounds were extracted 8 ACS Paragon Plus Environment

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into CH2Cl2 from the crosslinked product using a Soxhlet extractor. The residue was dried to constant weight at reduced pressure and room temperature to yield 0.96 g (46%) of a yellow crosslinked gel. As summarized in Table 2, yellow crosslinked gels were similarly obtained using linseed oil. Formation of Glass Fiber Composites by a Sheet Molding Process. In a typical example, a mixture of soybean oil (30 g, 35 mmol) and MDA-BMI (25 g, 70 mmol) were applied to a 11.4 cm x 22.8 cm 2 oz / ft2 (0.062 g/cm2) continuous strand glass fiber swirl mat having randomly oriented fibers, using a tongue depressor to spread the resin slurry in the center of the mat. The sample was placed between mylar sheets and loaded into a preheated compression mold shimmed to 1/16”. The mold was closed with a pressure of roughly 7 MPa and the material was held at 150 °C for 10 min. After opening the press and removing the sample, the sample was cooled and the mylar was peeled off. Excess glass that was not impregnated with the resin was trimmed from the sample, resulting in an approximately 11.4 cm x 11.4 cm composite sample.

Figure 3. 1H NMR (500 MHz) spectrum of the soybean oil used in this study. 9 ACS Paragon Plus Environment


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Results and Discussion Model “Ene” Reaction of N-Phenylmaleimide with Soybean Oil. Figure 3 presents the 1H NMR spectrum of the soybean oil used in this study. The broad resonance at 5.34 ppm corresponds to the methine proton at the glycerol junction and the olefinic protons along the fatty acid chains. The doublet of doublet resonances at 4.15 ppm and 4.31 ppm correspond to the four diastereotopic methylene protons alpha to the methine at the glycerol junction. The presence of the bisallylic methylene protons of the linoleic and α-linolenic chains are evident by the resonance at 2.77 ppm. The resonances at 2.31 ppm and 1.61 ppm are due to the methylene protons that are alpha and beta to the carbonyl carbon, respectively. The resonance at 2.04 ppm corresponds to the allylic protons. The remaining methylene protons of the long-aliphatic chains resonate as a strong multiplet at 1.26 ppm. The terminal methyl protons of each triglyceride arm resonate at 0.88 ppm. Since the integral of these methyl protons will not change before or after curing, the methyl resonance was used as an internal standard to calculate the number of double bonds in soybean and linseed oils.

Scheme 1. Two possible mechanisms for the reaction of a maleimide with plant oils.


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As outlined in Scheme 1, an enophile can react with the conjugated double bonds and allyl groups of triglycerides through both Diels-Alder and ene reaction mechanisms, respectively.30,31,38,39 Although no conjugated double bonds are detected by resonances in the region of 5.5 - 6.4 ppm, conjugated double bonds will form by an ene reaction of Nphenylmaleimide with the bisallylic group, and possibly by isomerization of the non-conjugated double bonds under the conditions used for curing. In addition, homopolymerization of Nphenylmaleimide and radical copolymerization of triglycerides and N-phenylmaleimide may occur at the same time. We therefore characterized the products of model reactions of soybean oil with N-phenylmaleimide in varying amounts at 150 °C for 24 h (Table 1). Based on the 1H NMR spectrum of this soybean oil, one molecule of soybean oil corresponds to approximately 4.6 carbon-carbon double bonds. Figure 4 presents the 1H NMR spectrum of the product of the model reaction using a 0.6 : 1 molar ratio of soybean oil and N-phenylmaleimide, which corresponds to approximately 2.8 double bonds per maleimide group. Although the complexities of soybean oil and its many possible reaction sites complicate the spectrum, the resonances are broader than those of the starting soybean oil in Figure 3, which indicates that a reaction occurred; a small resonance is detectable at 6.8 ppm for a small amount of unreacted N-phenylmaleimide. The succinimide product of the ene reaction should present a pair of doublet of doublets at approximately 2.7 ppm and 3.0 ppm due to the diastereotopic methylene protons, and a doublet at 3.3 ppm due to the methine proton.40 While the resonances of the succinimide methine and one of the methylene protons are hidden by the two DMI resonances, a characteristic doublet is detected at 2.93 ppm; in this case, the additional splitting is not resolved due to the broadening that is evident in all of the resonances.



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Figure 4. 1H NMR spectrum of the product of the model ene reaction of 0.6 : 1 soybean oil and N-phenylmaleimide in 1,3-dimethyl-2-imidazolidinone (DMI) at 150 °C for 24 h (entry 4 in Table 1). Although the narrow DMI methyl resonance at 2.8 ppm also overlaps the position of the broader bisallylic resonance, the lack of broadening at the baseline indicates that the bisallylic protons have been consumed in the ene reaction. The lack of the broad resonance at 2.8 ppm, and the remaining prominent resonance of the monoallylic group at 2.05 ppm demonstrates that the bisallylic groups were more reactive than the monoallylic groups in the ene reaction with Nphenylmaleimide. Reaction of the bisallylic groups will produce conjugated doubly bonds, which resonate at 5.3 ppm,41 and overlaps the methine of the glycerol linkage in the soybean oil, as well as other =CH resonances; nevertheless, comparison of the 1H NMR spectra in Figures 3 and 4 demonstrate that the resonances centered at 5.3 ppm have broadened and changed shape after reaction, evidently due to the formation of conjugated double bonds due to ene reaction of the bisallylic groups with N-phenylmaleimide. In spite of the formation of conjugated double 12 ACS Paragon Plus Environment

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bonds, no olefinic resonances due to the Diels-Alder adduct are detected at 5.82 ppm and 5.95 ppm, demonstrating that the only pericyclic reaction that has occurred is the ene reaction. All of the 1H NMR resonances of the reaction product in Figure 4 are broadened relative to those of the starting soybean oil in Figure 3 and N-phenylmaleimide, especially the aromatic resonances at 7.0-7.5 ppm.

The broad aromatic resonances may indicate that some N-

phenylmaleimide has oligomerized,42,43 although the reaction was performed in a nitrogen atmosphere and no free radical initiator was added. Nevertheless, we also did not add a free radical inhibitor to prevent free radical reactions initiated by hydroperoxides that may have formed by autoxidation44 of the “drying” plant oil; N-phenylmaleimide also has a very slight tendency to thermally self-initiate.43 However, the phenyl resonances in Figure 4 are not nearly as broad as those of poly(N-phenylmaleimide) produced by radical and

anionic

homopolymerizations,45 which resonate broadly over the range of 6.0 - 8.0 ppm. The spectrum in Figure 4 also lacks the very broad resonance from 3.0 to 5.0 ppm due to the methine protons of an oligo(N-phenylmaleimide) backbone. We therefore conclude that radical oligomerization of N-phenylmaleimide was minimal or nonexistent. Curing of Plant Oils with Bismaleimides. In order to potentially produce crosslinked materials, both soybean oil and linseed oil were cured with the three bismaleimides shown in Figure 2 in various ratios (Table 2) at 150 °C, as established for the model reaction of soybean oil and N-phenylmaleimide, but without added solvent. As established in the previous section, one molecule of this soybean oil corresponds to approximately 4.6 carbon-carbon double bonds. Similarly, one molecule of the linseed oil used corresponds to 6.0 carbon-carbon double bonds. Figure 5 presents the DSC thermograms of a 1:2 mixture of soybean oil and MDA-BMI. During the first heating scan, MDA-BMI melts at 158 °C, and is then followed by an exothermic curing



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reaction between soybean oil and the bismaleimide, with the peak maximum at 179 °C. This demonstrates that the curing reaction in bulk is dictated by the melting temperature and/or fluidity of the bismaleimide. No transitions are observed in the second heating scan, which confirms that the ene curing reaction was completed on the first heating scan. Although both soybean oil and linseed oil, as well as all three bismaleimides, are soluble in chloroform, methylene chloride, dimethyl sulfoxide, tetrahydrofuran and methanol, all of their cured products are insoluble in these and other common organic solvents. Therefore, the cured products are crosslinked.

Figure 5. Differential scanning thermograms (10 °C/min) of a 0.5:1 mixture of soybean oil and MDA-BMI, similar to entry P3 in Table 2. Table 3. Thermal and Mechanical Properties of Crosslinked Polymers Produced by Curing Plants Oils with Bismaleimides in Bulk at 150 °C for 45 min.a Table 2

Ene: Enophile

T5wt% T5wt%

Tg

E'28 °C

νe

Tensile 14


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Entry

a

(°C) in N2

(°C) in air

(°C)

(MPa)

(10-3 mol m3 )

Modulus (MPa)

P3

Soybean Oil: MDA-BMI

334

288

153

2150

84

brittle

P4

Linseed Oil: MDA-BMI

332

262

156

3200

126

brittle

P5

Soybean Oil: BMI-1700

366

353

12

488

11

-

P6

Linseed Oil: BMI-1700

376

342

10

488

14

-

P8

Soybean Oil: BMI-3000

428

363

-5

278

3

0.015

P10

Linseed Oil: BMI-3000

409

342

11

430

12

0.012

T5wt% is the decomposition temperature corresponding to 5 percent weight loss of the material;

Tg is the glass transition temperature measured by dynamic mechanical analysis (DMA); E' is the storage modulus measured by DMA; νe is the crosslink density (number of moles of network chains per unit volume of cured polymer) estimated from the rubbery plateau modulus. Table 3 summarizes the thermal and mechanical properties of selected samples from Table 2 involving both soybean oil and linseed oil. The corresponding TGA curves in nitrogen and air are presented in Figures 6 and 7, respectively. The thermosets based on BMI-3000 are more thermally stable than those based on BMI-1700, although BMI-1700 has a much more aromatic structure than BMI-3000; this indicates that the molecular weight of the oligomeric bismaleimide crosslink is more important than its chemical structure for the thermal stability of the crosslinked thermoset. Although the thermosets based on both oligomeric bismaleimides are more thermally stable than those based on the small molecule MDA-BMI, even the MDA-BMI thermosets are stable to 330 °C in nitrogen and 260 °C in air. Figure 6 demonstrates that rapid weight loss occurs when the cured thermosets are heated in nitrogen to temperatures of 320 °C to 520 °C. Above 550 °C, little weight loss occurs, indicating that stable char has formed. The



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char yields of P3 and P4 based on MDA-BMI are approximately 20 wt%, while those based on the oligomeric bismlaeimides are approximately 3 wt%. All of the cured thermosets are more stable in nitrogen than in air. The thermosets based on BMI-1700 and BMI-3000 are stable to 340 °C in air, and those based on MDA-BMI are stable to 260 °C in air. Therefore, BMI-1700 and BMI-3000 crosslinkers greatly improve the heat resistance of soybean oil and linseed oil, which are stable to 250 °C and 233 °C in air,46,47 respectively, whereas the MDA-BMI crosslinker provides only a slight improvement. In air, the cured thermosets decompose over three steps (Figure 7). The first maximum weight loss occurs at 365 °C. Since all of the samples exhibit weight loss at this temperature, with the greatest weight loss occurring for thermosets based on the small molecule MDA-BMI crosslinker, which should produce the densest crosslink structure, this indicates that the least thermally stable part of the thermoset involves the structure contributed by the plant oil. The second maximum weight loss occurs at temperatures between 440-460 °C, and the third maximum weight loss occurs at 530 °C to 570 °C. The thermosets based on the oligomeric bismaleimides (P5, P6, P8, P10) decompose over a narrower temperature range than those based on MDA-BMI (P3, P4).


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Figure 6.

Thermogravimetric analysis under nitrogen of the products produced by curing

soybean oil and linseed oil with MDA-BMI, BMI-1700 and BMI-3000 at 150 °C for 45 min; the samples correspond to entries in Tables 2 and 3.



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Figure 7. Thermogravimetric analysis in air of the products produced by curing soybean oil and linseed oil with MDA-BMI, BMI-1700 and BMI-3000 at 150 °C for 45 min; the samples correspond to entries in Tables 2 and 3. Glass fiber-reinforced composites with 45 wt% glass fibers were prepared by applying a 0.5:1 mixture of plant oil and bismaleimide crosslinker to a mat of continuous strand glass fibers, and then heating the materials in a sheet molding system with a press (~7 MPa) at 150 °C for 10 min. Figure 8 presents the DMA profiles for the glass fiber-reinforced composites from P3 and P4 based on MDA-BMI, and Table 3 tabulates the corresponding glass transition temperatures (Tg) and moduli.

The maximum tan δ values at 153 °C and 148 °C corresponds to the glass

transitions of the soybean oil- and linseed oil-based composites, respectively.

At room

temperature, the MDA-BMI composites are stiff with storage moduli of 2.2 GPa for P3 and 3.2 GPa for P4. Figure 9 presents the DMA profiles for the glass fiber-reinforced composites based on BMI-1700 and BMI-3000 crosslinkers, and Table 3 tabulates the corresponding glass transition temperatures and moduli. With these oligomeric crosslinkers, the maximum tan δ values corresponding to the glass transitions occur below room temperature at -5 to 12 °C, which demonstrates that the composites are flexible, rather than stiff.


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Figure 8. Dynamic mechanical analysis of the glass fiber-reinforced composites of soybean oil and linseed oil with MDA-BMI cured at 150 °C for 10 min; the resins used correspond to entries in Tables 2 and 3.



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Figure 9. Dynamic mechanical analysis of the glass fiber-reinforced composites of soybean oil and linseed oil with BMI-1700 (2:1 molar ration; P5 and P6) and BMI-3000 (0.5:1 molar ratio; P8 and P10) cured at 150 C for 10 min; the resins used correspond to entries in Tables 2 and 3. The glass transition temperatures and moduli of composites depend on the crosslinking density, as well as the direction of the glass fibers in the composites and the uniformity of the molded samples. From the DMA curves, the plateau of the elastic modulus in the rubbery state can be used to qualitatively compare the extent of crosslinking in composites made from the three different bismaleimides. The crosslink density (νe) is the number of moles of network chains per unit volume of cured polymer, and can be estimated from the rubbery plateau modulus using an equation from the statistical theory of rubber elasticity (eq. 1),48 νe = E'/3RT

(eq. 1)

in which E' is the tensile storage modulus obtained in the rubbery plateau, T is the temperature in K corresponding to the storage modulus value, and R is the gas constant. Although the equation is valid only for lightly crosslinked materials, it can be used to qualitatively compare the extent 20 ACS Paragon Plus Environment

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of crosslinking among polymers with similar structures. As summarized in Table 3, P4 has a higher crosslink density than P3, and the composite is therefore stiffer with a slightly higher glass transition temperature. Among the flexible composites, P8 has the lowest crosslink density and therefore the lowest glass transition temperature.

Figure 10. Tensile properties of the glass fiber-reinforced composites of soybean oil (P8) and linseed oil (P10) with BMI-3000 at 150 °C for 10 min; the samples correspond to entries in Tables 2 and 3. Using BMI-300 as an example of an oligomeric crosslinker, Figure 10 presents the room temperature tensile properties of the glass fiber-reinforced composites of plant oils cured with a bismaleimide at 150 °C for 10 min. (The MDA-BMI composites were too brittle for tensile



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testing.) There is very little difference between the composites based on soybean and linseed oils; their tensile moduli, tensile strengths and elongations at break are similar.

Conclusions Based on examples with soybean oil and linseed oil, plant oils can be cured with bismaleimides by the "ene" reaction at moderate temperature (150 °C) in bulk, without modifying the plant oil and without adding solvent or catalyst, to yield crosslinked bio-based thermoset polymers. Although the plant oils were cured with the bismaleimides at 150 °C over 45 min, glass-fiber reinforced composites with a range of mechanical properties were achieved by curing at 150 °C for only 10 min. Good mechanical and thermal properties were achieved even with less than 50% of crosslinker used relative to the number of allyl groups available. The properties of these crosslinked polymers depend on the nature of the bismaleimide crosslinker. The most thermally stable materials were produced with the highest molecular weight oligomeric bismaleimide, BMI-3000, with thermal stability to 340-360 °C in air, and 410-430 °C in nitrogen. Although not as thermally stable (260-290 °C in air and ~330 °C in nitrogen), the most highly crosslinked materials were achieved with the smallest bismaleimide, 1,1'-(methylene-di1,4-phenylene)bismaleimide (MDA-BMI). Glass-fiber reinforced composites based on MDABMI were therefore stiff with storage moduli of 2.2 - 3.2 GPa, and high glass transition temperatures of ~155 °C.

In contrast, the oligomeric bismaleimides produced flexible

composites with low elastic moduli of 0.3 – 0.5 GPa, and low glass transition temperatures of -5 to 10 °C. Therefore, the curing of plant oils with bismaleimides by the ene reaction provides a very simple method for producing bio-based thermoset polymers and reinforced composites with high thermal stability and a range of mechanical properties that are useful for commercialization,


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possibly using existing industrial processes. This technology enables incorporation of up to ~60 wt% unmodified plant oil, which is much higher than that of existing commercial thermoset composite materials.49

Author Information Corresponding Author * E-mail: [email protected], Tel.: (330) 972-6614

Notes. The authors declare no competing financial interest.

Acknowledgment is made to the National Science Foundation for support of this research through SBIR grants 1142327 and 1256123. We also acknowledge Designer Molecules (San Diego, CA) for their generous donation of the oligomeric bismaleimides. This contribution was identified by Session Chair Don S. Wardius (Covestro Inc.) as the Best Presentation in the corresponding session of “Sustainable Polymers, Processes & Applications” of the 2016 ACS Spring National Meeting in San Diego, CA.

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162x73mm (300 x 300 DPI)


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Moderate Temperature Curing of Plant Oils with Bismaleimides via the

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